1. Field of Invention
The invention relates to a method for performing corrective processing of a workpiece using gas cluster ion beam (GCIB) processing, and more particularly, to adjusting the surface profile of the workpiece.
2. Description of Related Art
Gas-cluster ion beams (GCIB's) are used for many applications, including etching, cleaning, smoothing, and forming thin films. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such gas clusters may consist of aggregates including a few to several thousand molecules, or more, that are loosely bound together. The gas clusters can be ionized by electron bombardment, which permits the gas clusters to be formed into directed beams of controllable energy. Such cluster ions each typically carry positive charges given by the product of the magnitude of the electron charge and an integer greater than or equal to one that represents the charge state of the cluster ion.
The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per individual molecule. The ion clusters disintegrate on impact with the workpiece. Each individual molecule in a particular disintegrated ion cluster carries only a small fraction of the total cluster energy. Consequently, the impact effects of large ion clusters are substantial, but are limited to a very shallow surface region. This makes gas cluster ions effective for a variety of surface modification processes, but without the tendency to produce deeper sub-surface damage that is characteristic of conventional ion beam processing.
Conventional cluster ion sources produce cluster ions having a wide size distribution scaling with the number of molecules in each cluster that may reach several thousand molecules. Clusters of atoms can be formed by the condensation of individual gas atoms (or molecules) during the adiabatic expansion of high pressure gas from a nozzle into a vacuum. A skimmer with a small aperture strips divergent streams from the core of this expanding gas flow to produce a collimated beam of clusters. Neutral clusters of various sizes are produced and held together by weak inter-atomic forces known as Van der Waals forces. This method has been used to produce beams of clusters from a variety of gases, such as helium, neon, argon, krypton, xenon, nitrogen, oxygen, carbon dioxide, sulfur hexafluoride, nitric oxide, and nitrous oxide, and mixtures of these gases.
One emerging application for GCIB processing of workpieces includes corrective spatial processing. Therein, a workpiece property, such as a film thickness, is spatially adjusted across the workpiece. By varying the GCIB dose between two separate locations on the workpiece, the film thickness, or other workpiece property, may be adjusted relative to each location. For example, as illustrated in FIG. 1, an upper surface 14 of a thin film 12 on substrate 10 may be planarized by selectively removing material from upper surface 14 using a GCIB. In the exploded view 20 of upper surface 14, an initial surface profile 16 may be flattened to produce a planar final surface profile 22 by selectively removing or etching material 18. The amount of material 18 to be removed from the upper surface 14 of thin film 12 directly affects the required dose required at each location on substrate 10 and, hence, the total amount of time needed for GCIB processing. Therefore, as the amount of material 18 increases, the throughput will decrease, and the processing time may become expensive or even prohibitive.